Spin-spin interactions in organic magnetoresistance probed by angle-dependent measurements.

The dependence of organic magnetoresistance (OMAR) on the orientation of the magnetic field has been investigated. In contrast with previous claims, a finite and systematic change in magnitude is observed when the orientation of the field is changed with respect to the sample. It is demonstrated that, to explain these effects, spin-spin interactions have to be included in the models previously suggested for OMAR. Dipole coupling and exchange coupling are introduced in combination with either an anisotropy of the orientation of the spin pairs or an anisotropy in the hyperfine fields.